1. Technical Field
The present invention relates to wireless communication apparatuses that receive signals via antennas.
2. Description of Related Art
A conventional wireless communication apparatus includes an antenna for receiving predetermined signals having a reference frequency, a resonance circuit that includes a capacitor and resonates with the antenna, a capacitance varying circuit that varies the capacitance of the capacitor of the resonance circuit, an oscillation circuit that applies a test signal to the resonance circuit, a voltage detection circuit that detects the output voltage of the resonance circuit, and a control circuit that controls the capacitance of the capacitor of the resonance circuit by controlling both the oscillation circuit and the capacitance varying circuit (see, for example, Japanese Patent Application Publication No. 2007-290664).
More specifically, the control circuit controls the oscillation circuit to vary the frequency of the test signal that is applied by the oscillation circuit to the resonance circuit. When the amplitude of the output voltage of the resonance circuit detected by the voltage detection circuit reaches its peak, the control circuit detects the then frequency of the test signal as a measured resonance frequency. Further, the control circuit controls the capacitance varying circuit to vary the capacitance of the capacitor of the resonance circuit, thereby bringing the measured resonance frequency into agreement with the reference frequency of the predetermined signals.
With the above configuration, when the capacitance of the capacitor of the resonance circuit is changed due to a change in temperature, it is still possible to keep the resonance of the resonance circuit with the antenna at a high level.
On the other hand, when the impedance of the antenna is changed due to a change in arrangement of metal members around the antenna, the resonance frequency of the antenna will deviate from the reference frequency of the predetermined signals. However, from the prior art, it is unknown how to bring the resonance frequency of the antenna back into agreement with the reference frequency of the predetermined signals in such cases.